Design Log DEC 019 poe transformer topology acf option a selected - thepinkmile/Enigma-NG GitHub Wiki
DEC-019 - PoE Transformer Topology: ACF (Option A) Selected
- Status: ✔ Adopted
- Date: 2026-04-05
- Category: Electrical
- Area: Power Module - T2 transformer, TPS23730 operating mode, input EMI filter
Decision
The Power Module retains the Active Clamp Flyback (ACF) topology with the Coilcraft POE600F-12LD
transformer. The TPS23730 operates in ACF mode, driving an external active clamp MOSFET complementary
to the primary switch. The Coilcraft transformer is procured order-direct from coilcraft.com and
supplied to JLCPCB as a consignment part.
No change is made to the T2 transformer, the TPS23730 mode configuration, or the active clamp circuit.
Full investigation detail: design/Electronics/Investigations/PoE_Investigation.md
Rationale
A thorough search confirmed that no ACF PoE isolation transformer for 60W / 36-57V input is stocked by any mainstream authorised distributor (DigiKey, Mouser, Farnell, Arrow). The Coilcraft POE600F family was co-developed with TI specifically for the TPS23730 reference design; no competing catalogue part exists.
The only standard-distributor alternative identified was the Bourns PDC060-FD20A12S - a standard flyback transformer available from DigiKey. Switching to this part (Option B) would have required:
- Disabling TPS23730
ACF_GD(ACF gate drive pin) - Removing the active clamp MOSFET and capacitor
- Adding an RCD clamp (3 SMD components)
- Verifying PSR auxiliary winding compatibility (unconfirmed)
After detailed EMI/EMC analysis, Option B was rejected on technical grounds. Key findings:
| Criterion | Option A (ACF) | Option B (Flyback) |
|---|---|---|
| Primary switching | ZVS - no hard dV/dt event | Hard switching - fast dV/dt |
| Drain spike at turn-off | None - energy recycled by clamp | Present - RCD clamp clips but does not eliminate |
| CM/DM conducted EMI | Low | 15-25 dB higher before input filter |
| Input EMI filter size | Smaller / fewer components | Larger / more complex |
| CISPR 32 Class B margin | Comfortable | Tight - may require design iteration |
| Efficiency | 88-92% | 85-90% |
| Extra dissipation | - | ~1-2W at full load |
| PSR aux winding verified | ✔ TI reference design | ⚠️ Unverified for Bourns PDC060 |
The Coilcraft direct-order model is accepted as a specialist procurement path, consistent with industry practice for catalogue magnetics.
Alternatives Considered
- Bourns PDC060-FD20A12S (Option B - flyback PSR): Rejected - hard-switching topology causes significantly higher conducted and radiated EMI, requires a larger input EMI filter, does not eliminate procurement complexity (shifts effort from transformer sourcing to filter design), and adds ~1-2W thermal load to an already thermally constrained module.
- Wurth WE-FB range: Rejected - entire range has a maximum input voltage of 36V (insufficient for PoE 36-57V input) and is keyed to Linear Technology LT-series controllers.
- Change PoE controller ecosystem: Not evaluated - would constitute a major redesign and is out of scope.